1. Field of the Invention.
The invention relates to a tensioning arrangement for a flextensional sonar transducer assembly. More specifically, the invention relates to a structure and method for tightening a band-type compression member around an elongate acoustical stack of piezoelectric crystals.
2. Description of the Related Art.
Piezoelectric elements, primarily crystals and ceramics, are employed in a variety of devices including crystal microphones, ultrasonic devices, accelerometers and oscillators. One of the most common uses of piezoelectric elements is in underwater sonar equipment in which a piezoelectric sonar transducer is stimulated by electrical signals to emit sonar signals which radiate out from the transducer. The sonar signals are reflected off of underwater objects and the reflected signals are then detected by the transducer, which in turn produces and delivers electrical signals carrying information about the underwater object.
Flextensional sonar transducers of the prior art employ a stack of piezoelectric transducer elements interspersed with electrically conducting plates for stressing the elements and for picking up electrical current produced by the elements; a prestressed compression band, made for example of a filamentwound material, wrapped about the piezoelectric stack; and an outer elliptically-shaped shell wrapped about the compression band. The stack of piezoelectric elements generally extends along the major axis of the ellipse defined by the outer shell. When an alternating voltage is applied to the conducting plates, the stack of piezoelectric elements is caused to be displaced in the direction of the major axis in proportion to the instantaneous value of the voltage. The vibration and displacement of the stack is transmitted to the shell which amplifies the vibration along the minor axis of the ellipse to produce the sonar signals. That is, as the stack expands to expand the major axis of the ellipse, the long walls of the ellipse perpendicular to its minor axis contract, and as the stack contracts to expand the long walls of the ellipse, vibration of the shell necessary to generate the sonar is produced. In an alternative arrangement of a flextensional transducer, a magnetostrictive element may replace the piezoelectric stack.
The elliptical shells used in flextensional transducers are typically preformed of filament-wound composites such as glass, reinforced plastic or aluminum. In order to incorporate the stack of piezoelectric elements in the shell, the shell is compressed along its minor axis by means of a press, and the piezoelectric stack is inserted into the shell to coincide with the major axis. Upon removal of the compressive force from along the minor axis, a residual force remains in the shell to retain the stack and apply a predetermined compressive stress thereto. Construction of the assembly in this fashion requires the piezoelectric stack and elliptical shell be prepared to close tolerances both to allow for easy insertion of the stack within the compressed shell, and to retain tight contact between the stack and the shell upon removal of the compressive forces.
The compression band allows for the application of a substantial prestress (compression) to the piezoelectric stack. The application of a compression stress to the stack by a wound compression band allows for the operation of the transducer in deep water. However, it is difficult to "wind in" as much prestress as desired due simply to the complication of winding the compression band while also maintaining the stack under stress.
When the transducer assembly is deployed in water, the increasing hydrostatic pressure with depth reduces the prestress on the stack, since the elliptical shell tends to be compressed along the minor axis thus removing shell pressure along the shell's major axis, and therefore along the major axis of the piezoelectric stack. Since tensional stress along the stack will cause it to crack and therefore be destroyed, a depth is eventually reached beyond which the transducer cannot drive without damage (the point where the forces on the stack in the major axis of the ellipse pass from compression to tension).
If however, the operational depth of the prior art transducers is exceeded, the hydrostatic pressure reduces the minor axis of the elliptical shell, and extends the major axis thereof, in a manner to exceed the prestress of the shell so that it "creeps" or moves (elongates) relative to, and then breaks away and becomes detached and decoupled from, the ends of the compression band and piezoelectric stack. When a prior art device using epoxy or the like to hold the shell in contact with the ends of the stack is subject to the deep water forces which tend to pull the shell away from the stack end pieces, sufficient tensional forces are produced thereon before the shell breaks away, to crack and destroy the crystals that make up the stack.
When "creep" and/or separation of the shell and stack occurs because of shell elongation, the shell does not return to its original position of coupling upon its removal from the high pressure, deep water environment. This is because the high hydrostatic pressure causes plastic creep, i.e., a permanent elongation-type deformation within the shell, and on return to ambient pressure, the shell remains somewhat elongated, and the drive stack can no longer efficiently couple acoustic energy therewith. The characteristics of the transducer assembly in this state are so changed that it is no longer usable at the same water depth. At deeper depths, the shell might be compressed enough along the minor axis to "squeeze" onto, and couple with, the stack to become operable. But each such use at the deeper depths causes further creep and decoupling of the shell apices from the stack, until eventually the assembly becomes inoperable at any depth (at least any depth of interest).